Merge Future branch into Master

[justinvp]

New APIs were added in the Future branch a while ago. It'd be nice if the changes were merged into Master so we could make the new APIs available in netcoreapp1.1.

https://github.com/dotnet/corefx/compare/future

Some examples:

• [Api-addition] Regex group should provide a Name property. corefx#1384 - Regex group should have a Name property
• Add ConcurrentDictionary GetOrAdd/AddOrUpdate overloads with generic arg corefx#1783 - Add ConcurrentDictionary GetOrAdd/AddOrUpdate overloads with generic arg
• Implement generic interfaces on Regex collections corefx#1756 - Implement generic interfaces on Regex collections
• Add async XLinq document/element loading and saving corefx#2436 - Add async XLinq document/element loading and saving
• Add HashSet<T> ctors with capacity corefx#2862 - Add HashSet ctors with capacity

[stephentoub]

https://github.com/dotnet/corefx/issues/2869

[karelz]

Dupe of #15058